The purposes of the formation of a chromate conversion coating on the surface of galvanized steel are to provide corrosion resistance, improve adhesion of coatings and for aesthetic reasons. The conversion coating improves adhesion of coating layers such as paints, inks, lacquers and plastic coatings. A chromate conversion coating is typically provided by contacting galvanized steel with an aqueous composition containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. Growing concerns exist regarding the pollution effects of the chromates and phosphates discharged into rivers and waterways by such processes. Because of high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion processes require extensive waste treatment procedures to control their discharge. In addition, the disposal of the solid sludge from such waste treatment procedures is a significant problem.
Attempts have been made to produce an acceptable chromate free conversion coating for galvanized steel. Chromate free pretreatment coatings based upon complex fluoacids and salts or metals such as cobalt and nickel are known in the art. U.S. Pat. No. 3,468,724 which issued to Reinhold discloses a composition for coating ferriferous and zinc metal which comprises a metal such as nickel or cobalt and an acid anion selected from the group sulfate, chloride, sulfamate, citrate, lactate, acetate, and glycolate at a pH from 0.1 to 4.
PCT Publication No. WO 85/05131 discloses an acidic aqueous solution to be applied to galvanized metals which contains from 0.1 to 10 grams per liter of a fluoride containing compound and from 0.015 to 6 grams per liter of a salt of cobalt, copper, iron, magnesium, nickel, strontium or zinc. Optionally, a sequestrant and a polymer of methacrylic acid or esters thereof can be present.
The formation of chromate free conversion coatings on the surfaces of other metals such as aluminum are also known. U.S. Pat. No. 4,921,552 which issued to Sander et al., discloses a non-chromate coating for aluminum which is dried in place and which forms a coating having a gravimetric weight from about 6 to 25 milligrams per square foot. The aqueous coating composition consists essentially of more than 8 grams per liter dihydrohexafluozirconic acid, and more than 10 grams per liter of water soluble acrylic acid and homopolymers thereof and more than 0.17 grams per liter hydrofluoric acid. The disclosure notes that it is believed that the copolymers of acrylic acid would also be effective, however, no examples were given. U.S. Pat. No. 4,191,596 to Dollman et al., discloses a conversion coating for aluminum which consists essentially of from about 0.5 to 10 grams per liter of a polymer of polyacrylic acid and esters thereof and from about 0.2 to 8 grams per liter of an acid selected from the group H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6 and H.sub.2 SiF.sub.6. The pH of the solution is less than about 3.5.
A process for applying a protective coating to aluminum, zinc and iron under substantially identical operation conditions is disclosed in U.S. Pat. No. 3,682,713 to Ries, et al. The coating consists essentially of from 0.1 to 15 grams per liter of complex fluorides of boron, titanium, zirconium and iron, from 0.1 to 10 grams per liter of free fluoride ions and from 0.5 to 30 grams per liter an oxidizing agent such as sodium m-nitrobenzene sulfonate. The solution has a pH of from 3.0 to 6.8 and is free of phosphoric acid, oxalic acid and chromic acid.
The use of allyl ether copolymers in non-analogous arts such as dust control, dispersants and water treatment is known. Japanese patent publication SHO 56-155692 entitled Method of Collecting Dust discloses the use of acrylic acid/polyethylene glycol monoallyl ether copolymers to treat the recirculating water in an aqueous dust collection system. U.S. Pat. No. 4,500,693 which issued to Takehara et al., discloses the use of copolymers composed of a methacrylic acid and an allylic ether monomer which are useful as scale preventing agents in cooling water systems and wet dust collection systems, aqueous slurry dispersants in inorganic pigments, cement dispersants, and builders and detergents.
U.S. Pat. No. 4,471,100 which issued to Tsubakimoto et al., discloses a copolymer of maleic acid and polyethylene glycol ether and its use as a cement dispersant, pigment dispersant, chelating agent and scale inhibitor.
U.S. Pat. Nos. 4,872,995 and 4,913,882 to Chen et al., and U.S. Pat. Nos. 4,861,429 and 4,895,620 to Barnett et al., disclose methods and uses for acrylic acid/polyethylene glycol allyl ether copolymers in aqueous systems such as cooling water systems and paper making systems, as felt conditioners or to inhibit calcium oxalate deposition.